Recently, backlights for liquid crystal displays have been increased in size, and with the increase in the size of the backlights, a lot of cold-cathode fluorescent lamps have come to be used per each backlight. Also in inverter circuits for liquid crystal display backlights, multi-lamp lighting circuits are used for lighting a large number of cold-cathode fluorescent lamps.
Conventionally, to light a large number of cold-cathode fluorescent lamps, one or a plurality of high-powered step-up transformers are used, as shown in FIG. 16, and the cold-cathode fluorescent lamps are connected to the secondary-side outputs of the step-up transformers via a plurality of capacitive ballasts, whereby the secondary-side outputs of the step-up transformers are shunted to light a lot of cold-cathode fluorescent lamps.
To implement the above construction, there are used two conventional methods: one not utilizing resonance in a secondary circuit, and the other utilizing resonance in the secondary circuit, which is becoming popular in recent years. Although they are not distinguished from each other in a simplified circuit diagram, they are distinguished from each other when described in detail with reference to a transformer equivalent circuit.
FIG. 17 shows another example of the multi-lamp lighting circuit. In the figure, leakage flux step-up transformers are provided for respective cold-cathode fluorescent lamps, and by making use of leakage inductance generated on the secondary side of each step-up transformer, that is, by resonating the leakage inductance and a capacitive component of the secondary circuit, a high conversion efficiency and the effect of reducing heat generation are obtained.
This technique is disclosed by one of the inventors of the present invention in Japanese Patent No. 2733817. In this example, the current flowing through each cold-cathode fluorescent lamp is varied depending on the influence of parasitic capacitance generated, for example, by wiring on the secondary side of a backlight, the aging of the cold-cathode fluorescent lamp, and the manufacturing errors. To stabilize the current, the lamp current of each cold-cathode fluorescent lamp is returned to the control circuit, whereby the output control of the inverter circuit is performed.
Further, there is another technique which does not provide a leakage flux step-up transformer for each of the individual cold-cathode fluorescent lamps, but as shown in FIG. 18 and FIG. 19, provides a plurality of secondary windings with respect to one primary winding to thereby consolidate leakage flux transformers, with a view to reduction of costs.
In addition, as the inverter circuit for a cold-cathode fluorescent lamp, there is a type which uses a piezoelectric transformer other than a winding transformer. In this type of inverter circuit, one cold-cathode fluorescent lamp is generally lighted by one piezoelectric transformer.
On the other hand, when a plurality of hot-cathode lamps are to be lighted by one inverter circuit, the multi-lamp lighting is made possible by using a shunt transformer (so-called a “current balancer”) as disclosed in Japanese Laid-Open Patent Publication (Kokai) Nos. Sho 56-54792, Sho 59-108297, and Hei 02-117098. Such a current balancer per se is known in the example of use thereof for lighting hot-cathode lamps. Further, the impedance of hot-cathode lamps is very low, and the discharge voltage thereof is approximately 70 V to several hundreds of volts, which makes it unnecessary to pay much attention to the adverse influence of parasitic capacitance generated around each discharge lamp. Therefore, it is easy to apply the current balancer to the hot-cathode lamps.
Further, in this method, when one of the connected hot-cathode lamps is unlighted, an excessive voltage is generated at a terminal of a current balancer associated with the unlighted hot-cathode lamp, so that when hot-cathode lamps are partially unlighted, there is no other choice but to interrupt the circuit. Accordingly, the current balancer could not be put into practical use as a single device unless several countermeasures to the problem are taken beforehand. Moreover, the current balancer itself was conventionally large in size.
On the other hand, it is considered in principle that the current balancer can be similarly applied to parallel lighting of cold-cathode fluorescent lamps. However, many of the proposals which have been made are unstable, and no example of practical use has appeared for a long time period since the early days of the cold-cathode fluorescent lamp. Further, although the application of the current balancers to cold-cathode fluorescent lamps was experimentally possible, the size of the current balancer was too large for practical use. This is for the following reason:
It is considered that the parallel lighting of cold-cathode fluorescent lamps can be performed, for example, by a circuit configuration shown in FIG. 20. A typical example of disclosure is Republic of China patent No. 521947. In this example, ballast capacitors Cb are arranged in series with respective cold-cathodes DT, for current shunting, and a current balancer Tb is combined with the above arrangement, for obtaining the current-balancing effect.
As represented by the Republic of China patent No. 521947, it has been considered that the reactance of the current balancer is required to have a value well above the impedances Z1 and Z2 of cold-cathode fluorescent lamps, as calculated by the following equation:
Assuming that M represents the mutual inductance between L1 and L2, L1=L2=MV=(Z1+jωL1)·j1−jω·M·j2  1V=(Z2+jωL2)·j2−jω·M·j1  2From the above equations 1 and 2,{Z1+jω(L1+M)}·j1−{Z2+jω(L2+M)}·j2=0
                              j          2                =                                                                              Z                  1                                +                                  j                  ⁢                                                                          ⁢                                      ω                    ⁡                                          (                                                                        L                          1                                                +                        M                                            )                                                                                                                    Z                  2                                +                                  j                  ⁢                                                                          ⁢                                      ω                    ⁡                                          (                                                                        L                          2                                                +                        M                                            )                                                                                            ·                          j              1                                =                                                                      Z                  1                                +                                  2                  ⁢                                                                          ⁢                  j                  ⁢                                                                          ⁢                                      ω                    ·                                          L                      1                                                                                                                    Z                  2                                +                                  2                  ⁢                                                                          ⁢                  j                  ⁢                                                                          ⁢                                      ω                    ·                                          L                      1                                                                                            ·                          j              1                                                  3      Compared with Z1 and Z2, if 2ωL is sufficiently large, evenwhen Z1≠Z2,j1≈j2.

Further, in the case of the circuit configuration shown in FIG. 20, since the major part of the current-shunting effect is entrusted to the ballast capacitors Cb, it is possible to exhibit the current-shunting effect irrespective of the magnitude of the reactance of the current balancer Tb. In this case, the ballast capacitors Cb are essential, and the effect of causing lighting of discharge lamps C is obtained by a combination of a high voltage caused to be generated by a transformer at the immediately preceding stage, and the operation of the ballast capacitors Cb.
Further, in these proposals, the impedances of the cold-cathode fluorescent lamp are regarded as pure resistances based on a theory shown by the above equation and FIG. 20. More specifically, the impedances are determined by the VI characteristic (voltage-current characteristic) of the cold-cathode fluorescent lamp, and regarding the impedances as pure resistances, a reactance sufficiently larger than the impedances of the cold-cathode fluorescent lamp is set, whereby variation in the impedances of the individual cold-cathode fluorescent lamps is corrected.
More specifically, the reactance of the current balancer is set with a view to correction of variation in the impedances of the individual cold-cathode fluorescent lamps. Although it cannot be said that the theory is false, the reactance set as above does not reflect a minimum required reactance value. In this case, since the current balancer is provided for the purpose of correcting variation in the impedances of the individual cold-cathode fluorescent lamps, a considerably large reactance (mutual inductance) is required. Therefore, so long as the inductance is determined based on the theory, an inductance value required for the current balancer has to become excessive, and further, the current balancer inevitably has to be made fairly large in outside dimensions.
Inversely, if the outside dimensions of a current balancer are to be reduced to meet with the market demands, the effective permeability of a core material of the transformer is lowered, so that when the required inductance determined by the above equation is to be secured, the coil has to be formed by a large number of turns of an extra fine wire. However, this results in increased distributed capacitance, thereby causing a decrease in the self-resonance frequency of the current balancer, so that the current balancer loses its reactance. This can lead to degradation of current-balancing capability of the current balancer. As a result, the current balancer cannot properly shunt current so that the imbalance of currents is caused.
Since cold-cathode fluorescent lamps used for a liquid crystal display backlight are discharge lamps, they have a negative resistance characteristic. This characteristic is drastically changed, when the cold-cathode fluorescent lamps are mounted on the liquid crystal display backlight. However, originally, the negative resistance characteristic of each cold-cathode fluorescent lamp in the mounted state is not controlled, and hence e.g. when lots of liquid crystals are changed during mass production, various problems are liable to occur. Moreover, those skilled in the art have almost no recognition concerning the negative resistance characteristic of the liquid crystal display backlight. In view of the above circumstances, when small-sized shunt transformers are used, it has been considered essential to insert shunt capacitors Cb in series by way of precaution have been considered essential, in order to prevent occurrence of defective products during mass production.
Although the shunt capacitors Cb can be dispensed with, in this case, the outside dimensions of the shunt transformer have to be made sufficiently large. An increase in configuration leads to an increase in the self-resonance frequency of the coil having the same inductance value. In other words, the commercialization of shunt transformers has been insufficient or obstructed until the present invention has been made, mainly due to incomplete disclosure of details of the techniques.
Further, in the example of the conventional current balancer, saturation of the core, which is caused by imbalanced currents in the current balancer, for example, when one of the discharge lamps is unlighted, is regarded as harmful, and hence the saturation is detected by additionally providing a winding in the shunt transformer, for detection of abnormality of the circuit. If abnormality of the circuit is detected, operation of the circuit is blocked.
When a large number of discharge lamps are to be simultaneously lighted by the conventional inverter circuit for a discharge lamp, the discharge lamps cannot be connected to each other simply in parallel with each other even if they have the same load characteristics. This is because the discharge lamp has a characteristic that when the current flowing therethrough is increased, the voltage thereof is decreased, that is, a so-called negative resistance characteristic, and hence even if a plurality of discharge lamp loads are connected in parallel, only one of them is lighted, while all the others are unlighted.
To cope with the above problem, in the multi-lamp lighting circuit, as shown in FIG. 16, a method of shunting the output of the step-up transformer on the secondary winding side using capacitive ballasts is generally employed. However, the circuit for shunting the output of the step-up transformer using the capacitive ballasts is a simplified circuit, but suffers from the following various problems, which will be described hereinafter with reference to FIG. 16.
In an inverter circuit for cold-cathode fluorescent lamps, shown in FIG. 16, assuming that the cold-cathode fluorescent lamps have a length, for example, of approximately 300 mm, the discharge voltage of each cold-cathode fluorescent lamp is generally approximately 600 V to 800V. In this circuit, when the discharge current is stabilized by using the capacitive ballasts, the reactance of the capacitive ballasts are inserted in series with respect to the discharge lamps, so that a voltage obtained by adding up the voltage of the cold-cathode fluorescent lamp and a voltage applied to the capacitive ballasts comes to 1200 V to 1700 V. The thus obtained voltage is the voltage of the secondary winding of the step-up transformer, and hence a high voltage of 1200 V to 1700 V is continuously applied to the secondary winding of the step-up transformer, which causes various problems.
One of the problems is electrostatic noise irradiated from a conductor having a voltage of 1200 V to 1700 V, which requires electrostatic shielding as a countermeasure against the radiation noise.
The above high voltage induces generation of ozone. The ozone enters metal portions via soldered portions of the secondary winding or pin holes of the same. This causes metal ions, such as copper ions, to be generated, which move to enter plastics of winding bobbins of the transformer, sometimes lowering the withstand voltage of the winding bobbin.
Further, the metal ions move along the secondary winding, so that the secondary winding can be sometimes burned due to inter-layer short circuits (layer short circuits) caused by the metal ions.
That is, the continuous application of a high voltage to the secondary winding brings about serious problems concerning the service life and management thereof since the above-described troubles occur as changes due to aging of the products after shipping thereof.
As a method free from the problems as described above, there is proposed a method of providing a leakage flux step-up transformer for each cold-cathode fluorescent lamp to stabilize lamp currents flowing through the cold-cathode fluorescent lamps by ballast effects brought by the leakage inductances of the step-up transformers, and resonating the leakage inductances with the capacitive component of the secondary circuit, to thereby obtain high conversion efficiency (Japanese Patent No. 2733817) as shown in FIG. 17. The discharge voltages of the cold-cathode fluorescent lamps directly become equal to the voltages of the secondary windings of the leakage flux step-up transformers, which enables the burden of the voltages of the secondary windings to be reduced. As a consequence, it is possible to drastically reduce the aging and occurrences of burnouts.
In this method, however, it is necessary to provide a leakage flux transformer and a control circuit for each of cold-cathode fluorescent lamps, which brings about the problems of increases in the circuit size and the manufacturing costs.
According to the above method of circuit configuration, it is possible to eliminating variation in lamp currents flowing through cold-cathode fluorescent lamps by detecting a lamp current flowing through each cold-cathode fluorescent lamp and stabilizing the lamp current by controlling an associated drive circuit of the transformer, and maintain the luminance of a liquid crystal display backlight at an averaged and constant level until just before the end of service life thereof. Therefore, the circuit system is in widespread use as an excellent system, in spite of the problem of costs.
Therefore, as acceptable compromise for improvement the above method in respect of costs thereof, an attempt has also been made to reduce costs of transformers, by assembling a plurality of leakage flux transformers, for example, to provide one primary winding with two secondary windings, or put together two leakage flux transformers using one core, as shown in FIGS. 18 and 19.
In this method, however, it is not possible to control individual electric currents flowing through a plurality of cold-cathode fluorescent lamps connected to a transformer, so that only one current control can be carried out on the primary winding of the transformer. Further, when there occurs an imbalance between lamp currents flowing through the cold-cathode fluorescent lamps connected to the secondary windings formed as an assembly, it is almost impossible to make the currents balanced with each other.
Although the above description has been given of the winding transformer, the same problem occurs with an inverter circuit using a piezoelectric transformer.
The piezoelectric transformer is sometimes fractured when a step-up ratio thereof is increased to obtain a high voltage. Therefore, it is not practical to light a plurality of cold-cathode fluorescent lamps by increasing the step-up ratio, and shunting electric current into a plurality of cold-cathode fluorescent lamps using the capacitive ballasts.
Accordingly, in general, one piezoelectric transformer can be connected to only one cold-cathode fluorescent lamp, and hence the use of a piezoelectric inverter circuit has been limited.
On the other hand, an attempt has been made to apply the use of current balancers, which have been realized in hot-cathode lamps, to cold-cathode fluorescent lamps to thereby simultaneously light approximately two to four lamps cold-cathode fluorescent lamps, while suppressing variation in currents.
However, the shunt capacitors Cb increases voltage applied to the secondary windings of transformers, causing acceleration of aging thereof, so that it is desirable to eliminate the shunt capacitors if possible. When a large number of cold-cathode fluorescent lamps are arranged in parallel for multi-lamp lighting, in most cases, the effect thereof is very unstable, and it sometimes becomes impossible to obtain the shunting and balancing effects all of a sudden, with a different construction of a backlight or a different type of cold-cathode fluorescent lamps. To overcome this problem, a shunt capacitor Cb also serving as a ballast capacitor is provided in series with each fluorescent lamp so as to enable all the cold-cathode fluorescent lamps to be lighted even when the balancing effect is lost.
On the other hand, in the case of a shunt transformer for hot-cathode lamps, the shunting and current-balancing effects can be obtained without provision of shunt capacitors. This is because the shunt transformer can be relatively large in size since a large space for containing the shunt transformer can be provided, and it is desired that the core is prevented from being saturated by the imbalance of currents flowing through the shunt transformer, when one or some of hot-cold-cathode fluorescent lamps are unlighted.
Further, in the hot-cathode lamp, in general, there is a large voltage difference between a constant discharge voltage and a discharge starting voltage, and particular operation is required at the start of discharge. This necessitates additional operation of causing lighting of hot-cathode lamps by some kind of means.
The same applies to the lighting circuit for lighting cold-cathode fluorescent lamps, and it is necessary to perform operation of causing lighting of cold-cathode fluorescent lamps by some kind of means.
In the case of a circuit shown in FIG. 20, the effect of causing lighting of cold-cathode fluorescent lamps C is entrusted to the operation of the ballast capacitors Cb connected in series to the respective cold-cathode fluorescent lamps C, whereby the major shunting effect is obtained. In this method, however, similarly to the conventional inverter circuit, a high voltage is continuously generated in the secondary winding. Therefore, the problem of continuous application of a high voltage to the secondary winding is not alleviated.
As described above, it is desired to eliminate the shunt capacitors Cb, if possible, since they increase the voltage applied to the secondary winding, and accelerates aging. However, to guarantee a stable shunting effect while eliminating the shunt capacitor Cb, it is essential to control voltage-current characteristics observed as the result of mutual operation between the cold-cathode fluorescent lamp and a conductor (also serving as a metal reflector, in general) close to the cold-cathode fluorescent lamp.
Particularly, it is necessary to guarantee a negative resistance characteristic obtained from the voltage-current characteristics as a specification value. However, those skilled in the art have not recognized the necessity of controlling such a negative resistance characteristic from the early days of the liquid crystal display backlight up to the present time, so that an adequate reactance value that guarantees a stable shunting effect is obscure. Therefore, the shunt capacitors Cb have been indispensable, and when the capacitors Cb are eliminated, it is impossible to avoid increasing the shunt transformer so as to cause the shunt transformer to have a sufficient and excessive reactance value.
Further, reduction of the size of a shunt transformer based on the excessively set reactance value makes the self-resonance frequency of the shunt transformer too low, which impairs the effect of reactance related to shunting, so that the shunting effect is lost. As a consequence, the shunt capacitors Cb become indispensable again. Thus, the process goes round in circles to get nowhere.
Further, as a means for protection in case of failure of lighting due to abnormally occurring in one or some of discharge lamps, there has been conventionally provided a winding for detecting distorted current caused by magnetic saturation of the current balancer, for detection of abnormality. However, the protecting means has no operation or effect of protecting the shunt transformer itself.
Further, the conventional method of detecting abnormality is based on detection of deformation of the waveform of magnetic flux generated in the current balancer, and a means of the detection is not simple.
Further, to increase the size of the shunt transformer so as to prevent the saturation of the shunt transformer inversely leads to an increase in core loss caused by the saturation of the shunt transformer. This has caused generation of a considerable amount of heat.
Furthermore, the cold-cathode fluorescent lamp, which has a high constant discharge voltage, is largely influenced by the parasitic capacitance generated in nearby associated circuit components and wiring connected thereto, so that if the parasitic capacitances occurring in wiring between an inverter circuit and cold-cathode fluorescent lamps are different, imbalance in currents flowing through the cold-cathode fluorescent lamps is caused.